1. Field of the Invention
The present invention relates to polymer films. In particular, the present invention relates to medium density polyethylene compositions and polymer films blown from such polyethylene compositions and having improved impact and excellent mechanical properties and good processability. The invention also concerns a process for the production of the polymer compositions.
2. Description of Related Art
There is a growing demand for thinner films (downgauging) due to environmental aspects and cost reduction. Since film strength and performance in packaging lines should be maintained in spite of the downgauging, stiffer films with high mechanical properties are needed to meet future demands.
The polyolefin materials used for blown film applications can be divided into the following five groups of materials:
1) Low Density Polyethylene (LDPE) having broad molecular weight distribution (MWD) and produced by radical polymerisation in one reactor;
2) Linear Low Density Polyethylene (LLDPE) having narrow MWD and produced by polymerization in the presence of Ziegler catalysts in one reactor
3) High Density/Medium Density Polyethylene (HD/MDPE) having broad MWD and produced by polymerization in the presence of Cr catalysts in one reactor
4) High Density Polyethylene (HDPE) having a bimodal (broad) MWD) and produced by polymerization in the presence of Ziegler catalysts in two reactors in series (cascade)
5) Linear Low Density Polyethylene (LLDPE) having a bimodal (broad) MWD and produced by polymerization in the presence of Ziegler catalysts in two cascaded reactors.
The market for PE films is continuously developing improved solutions to packaging requests based on the above type of materials.
Unimodal LLD materials (point 2 above) used in film application, typically having density lower than 926 kg/m3, exhibit good mechanical properties (such as tear strength or dart drop). There are applications, however, where their stiffness is not high enough. In those applications materials with higher density are used. When the density has been increased to increase the stiffness, the mechanical properties have suffered in the trade-off.
Bimodal LLD film materials (point 5 above) have a superior balance between mechanical properties and stiffness, as well as a superior processability compared to a unimodal LLD. This type of material has been described e.g. in a Finnish Patent Application No. 945926. Some other bimodal LLDPE-type materials are disclosed in EP Patent Applications Nos. 0 492 656, 0 691 367, 0 773 257 and 0 773 258.
However, there are applications where an even higher stiffness is required. There are bimodal high density film materials available (point 4 above) with a high stiffness. This kind of material is discussed e.g. in EP Patent Application No. 0 691 353. Unfortunately, a reduction in the mechanical properties is then observed.
One problem associated with the bimodal (or multimodal) polyethylenes is the inherent heterogeneity of the material, due to the fact that it consists of components having very different molecular weights (or melt flow rates) and comonomer contents (or densities, or degrees of branching). The problem associated with the differences in molecular weight is discussed e.g. in an article by Bxc3x6hm, Enderle and Fleissner (Industrial Synthesis of Bimodal Polyethylene Grades with Improved Properties), where it is stated (on page 355) that problems are to be expected when polymers having a ratio between their molecular weights higher than 10 are mechanically blended. The problems associated with the different degree of branching (or different density) are studied by Morgan, Hill, Barham and Frye in a recent article (Liquid-Liquid Phase Separation in Ternary Blends of Linear Polyethylene with Two Ethylene Butylene Copolymers, Polymer, Vol. 38, No. 8, pp. 1903-1909, 1997). The authors show that phase separation may occur when linear PE is blended with an ethylene-butene copolymer (e.g. FIG. 1 and FIG. 4) having a relatively low degree of branching. It is also stated in article by Alamo, Kim, Mandelkem, Galante, Wignall, Londono and Stehling (The Cocrystallization of Mixtures of Homopolymers and Copolymers: Polyolefins, Polym. Mater. Sci. Eng., Vol. 78, pp. 213-214, 1998, page 213) that when linear PE is blended with branched PE having a branch concentration higher than 8 mol-%, phase separation in the melt occurs.
The consumption of MDPE for blown film is increasing and the market is typically supplied by unimodal Cr-based products. These materials are extensively used in coextruded films for stiffness-improvement, but give a relatively small contribution to other physical properties like impact required by the packaging. The processability on a film blowing line as well as the physical properties of the final film are very much dependent on the polymer structure, especially MWD and SCBD.
Thus, as apparent from the above, the available materials for film blowing give limited alternatives in terms of a balance between stiffness and other mechanical properties.
It is an object of the present invention to eliminate the problems of the prior art and to provide novel blown polymer films.
It is another object of the present invention to provide a process for producing suitable polyethylene materials for production of films, in particular blown films.
These and other objects, together with the advantages thereof over known processes and products, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
As mentioned above, the general belief in the art has been that if increased stiffness is sought, then some of the mechanical properties have to be sacrified by increasing the density of the material. However, contrary to this belief, it has now surprisingly been found that if the material with the higher density is bimodal and the increase of the density is made in a specific way, the mechanical properties remain unaffected while the stiffness is increased. The outcome is a material with mechanical properties equal to those of the LLD but with improved stiffness.
In particular, the present material comprises a bimodal medium density polyethylene with
i. 30 to 50 % by weight, preferably 35-45 %, and in particular 37-43 %, of a low molecular weight portion having a density of at least 960 kg/m3 and an MFR2 greater than 100g/10 min and a low comonomer content and
ii. 70 to 50% by weight, preferably 65 to 55%, and in particular 63-57%, of a high molecular weight portion having a density calculated to be in the range of 890 to 920 kg/m3 and a high comonomer content,
the present polyethylene having a density of about 920-945 kg/tm3, in particular 925-940 kg/m3, and a melt flow rate in the range of MFR21 5 to 50 g/10 min. The comonomer content of the high molecular weight portion is at least about twice higher, preferably at least about 3 times higher than in the low molecular weight portion.
The present bimodal MDPE material has an elongational viscosity which increases with increasing elongation rate. Therefore, it is particularly well suited for the production of blown films. The films according to the present invention exhibit, depending on density and MFR of the composition and on the film blowing conditions, a tensile strength in transverse direction of 15 MPa or more, and a 1% secant modulus in machine direction of at least 300 MPa and in transverse direction of 400 MPa or more.
The present compositions are preferably produced by polymerizing or copolymerizing ethylene in a reactor cascade formed by at least two reactors operated with different amounts of hydrogen and comonomers to produce a high molecular weight portion in one of the reactors and a low molecular weight portion in another.
The blown films exhibit a tensile strength in transverse direction of at least 15 MPa; 1% secant modulus in machine direction of at least 300 MPa and in transverse direction of at B least 400 MPa; and, when run to a film having a thickness between 35-45 xcexcm in a machine operating with a low neck, a dart drop of at least 5 g/xcexcm.
The process of producing a blown polyethylene film according to the present invention comprises the steps of producing a polyethylene composition in the presence of a catalytic system of an ethylene polymerizing catalyst and cocatalyst in a multistage reaction sequence consisting of successive polymerization stages where at least one stage is carried out in a gas phase by subjecting ethylene, optionally hydrogen and comonomers to a first polymerization or copolymerization reaction in a first reaction zone or reactor to produce a first polymer; recovering the first polymer and the catalyst therein from the first reaction zone; feeding the first polymer into a second polymerization zone or reactor; feeding additional ethylene and optionally hydrogen and comonomers to the second reaction Zone; subjecting the additional ethylene and optionally additional monomer(s) and hydrogen to a second polymerization reaction in the presence of the first polymer and the catalyst therein to produce a second polymerization product having an MFR21 of less than 50 g/10 min; and recovering the combined polymerization product from the second reaction zone, and blowing the thus obtained polyethylene composition into a film.
Considerable advantages are obtained by means of the present invention. Thus, the present invention provides a product having improved tear resistance as well as excellent film homogenity, compared to conventional Cr based materials at the same stiffness levels. Further, the product has an improved stiffness/impact balance and higher stiffness, allowing for downgauging and increased downstream converting speeds.
The novel MDPE material is particularly suitable for film blowing, with an improved processability allowing it to be run on a variety of film lines (LD, LLD and HD lines) and coextrusion film lines.
Next, the invention will be more closely examined with the aid of the following detailed description and with reference to the attached drawings.